1. Field of the Invention
This invention relates generally to a method and system for calibrating and characterizing image capture devices using a calibrated display of known colorimetric properties. More particularly, this invention relates to image capture devices using digital imaging technology.
2. Description of the Prior Art
Conventional methods for digital or electronic color image capture devices require the characterization and calibration of each device in order to produce digital values that have a known relationship to the original scene or image. This calibration or characterization data can be stored in a data file called a xe2x80x9cprofilexe2x80x9d describing the unique color capture characteristics of that specific device. Unit to unit variances in image capture devices, especially in the charge-coupled devices (CCD""s) in consumer digital cameras, means that a single xe2x80x9cgenericxe2x80x9d profile for one class or model of that device may not be accurate enough for good color imaging results. A unique profile for each unique image capture device is best for assuring that the device provides adequate color accuracy.
With respect to image capture devices, (such as a digital camera, for example), calibrating an image capturing device can be difficult due to aging of a system or the effect of a dusty environment. An imaging system typically suffers problems such as image defects or distortion. Further, it can be difficult to find a method that is simple enough for most end-users to implement. In addition, further problems can exist such as photo-response non-uniformity (PRNO) of the CCD to a light source, or distortion of the mirror or lens.
Aside from variances in CCD""s and other sensors, one of the reasons for the unit-to-unit variability among image capture devices is that the spectral sensitivities or responsivities of the image capture device may not be colorimetric. The spectral responses often are not a linear transformation of the CIE standard observer color matching functions. This property can inherently produce errors in the values that describe the captured color image. These errors are often known to those skilled in the art as instrument metamerism. In simple terms this means that two colors typically seen as different by a human observer are captured or recorded as the same set of values by the image capture device. Conversely, two colors that are typically seen as a match, identical, are captured and recorded by the device as two different sets of values. Also, due to manufacturing variations, the spectral responsivity or sensitivity of the various xe2x80x9ccolorxe2x80x9d channels of the image capture device may vary from device to device. Additionally, design criteria and production variations result in different relationships between the amount of xe2x80x9cscenexe2x80x9d or original spectral radiance and recorded or captured color value. This is commonly known to those skilled in the art of imaging science as the Tone Reproduction, and is also known as the input/output transfer function or xe2x80x9celectroopticxe2x80x9d transfer function. All of these factors, and many more, require that the image capture device be characterized and calibrated so the captured digital values are a useful representation of the color in the original object or scene.
This problem is widely recognized and several national and international standards organizations have defined a standard set of test patterns for use in the calibration and characterization of imaging devices. One of the most popular of these test patterns or targets are the IT8 (ANSI-IT8.7/1-3) series and the SCID (ISO12640:1995) and the MacBeth Color Checker. These patterns generally consist of an array of color patches or areas that have known CIE XYZ tristimulus values or CIE L*a*b* values or even perhaps spectral reflectance factor properties.
Although the present art is generally adequate for the process of calibration and characterization of image capture devices, the test patterns are often expensive to produce and of fixed colors. In some cases the test patterns are so large (SCID) that usage can be extremely difficult on desktop printers. Another disadvantage is that the colors are standardized and therefore fixed, and such fixed set of colors or color patches may not be optimum for the calibration and/or characterization of any particular image capture device.
Thus, there is a present need for a more accessible, affordable and simple method and system that can be used by consumers to calibrate and characterize image capture devices. Further, it would be advantageous for the method and system to calibrate and characterize image capture devices with relatively good quality color representation and with minimum overhead cost. Still further, the method and system would be relatively easy to use and would provide a flexible calibrated colorimetric color system which is well suited to various image capture devices and can be supported with minimum cost to software and hardware vendors.
The present invention provides, in a first aspect, a method for calibrating image capture devices from a calibrated display of known calorimetric properties. It is therefore a primary object of this invention to provide a method and system for compensating the aforementioned problems. The present invention provides a method for calibrating or characterizing an image capture device providing feedback from a known flexible calibrated display device. This feedback can either be remote or a local.
The present invention provides, in a first aspect, a method for calibrating and characterizing an image capture device, including the steps of first, calibrating a display device to generate a calorimetrically calibrated display. Next, displaying at least one reference image on the calibrated display to generate a calibrated reference image with known colorimetric properties. Then, capturing at least one image from an image capture device to be calibrated, to generate a captured image. Next, comparing the captured image with the calibrated reference image. Thereafter, computing a relationship between the captured image and the calibrated reference image to generate information on differences, and finally, writing a profile describing the relationship such that the profile describes the colorimetric characteristics of the image capture device.
In another aspect, the present invention provides a system for calibrating and characterizing an image capture device. The system includes, display means for displaying calibrated reference images, image capture means for capturing at least one image from an image capture device; processing means to compute a calorimetric profile based on a relationship between captured images and calibrated images, such that the profile describes the colorimetry of the image capture device.